The invention relates to the carbonylation of ethylene using carbon monoxide.
The carbonylation of ethylene using carbon monoxide in the presence of an alcohol or water and a catalyst system comprising a Group VIII metal, e.g. palladium, and a phosphine ligand, e.g. an alkyl phosphine, cycloalkyl phosphine, aryl phosphine, pyridyl phosphine or bidentate phosphine, has been described in numerous European patents and patent applications, e.g. EP-A-0055875, EP-A-04489472, EP-A-0106379, EP-A-0235864, EP-A-0274795, EP-A-0499329, EP-A-0386833, EP-A-0441447, EP-A-0489472, EP-A-0282142, EP-A-0227160, EP-A-0495547 and EP-A-0495548. In particular, EP-A-0227160, EP-A-0495547 and EP-A-0495548 disclose that bidentate phosphine ligands provide catalyst systems which enable higher reaction rates to be achieved.
A problem with the previously disclosed catalyst systems is that, although relatively high reaction rates can be achieved, the catalyst dies off quickly which necessitates the frequent replenishment of the catalyst and hence results in a process which is industrially unattractive.
WO 96/19434 discloses a particular group of bidentate phosphine compounds which can provide remarkably stable catalysts which require little or no replenishment and the use of such bidentate catalysts in a process for the carbonylation of ethylene with carbon monoxide.
It has now been found that, when used in the process for the carbonylation of ethylene with carbon monoxide, the activity and life of catalyst systems based on such phosphine compounds are very sensitive to the relative amounts of ethylene and carbon monoxide in the gaseous phase of the reactor. This is counter to the common teaching in the art which generally does not express any preference for the relative amounts of these reactants.
Surprisingly therefore, it has now been found that the activity and the life of the catalyst, when used in a liquid phase carbonylation process, can be significantly improved by using a high molar ratio of ethylene to carbon monoxide in the gas in contact with the liquid phase.
Accordingly, the present invention provides a process for the carbonylation of ethylene in a liquid phase, which process comprises
(i) forming a gaseous phase from an ethylene feed stream and a carbon monoxide feed stream;
(ii) contacting the gaseous phase with a catalyst system within the liquid phase containing a source of hydroxyl groups, said catalyst system comprising palladium, or a compound thereof, and a phosphine ligand together with a source of anions; and
(iii) reacting the ethylene with the carbon monoxide in the presence of the source of hydroxyl groups and of the catalyst system
characterised in that the ethylene feed stream and carbon monoxide feed stream provide a molar ratio of ethylene to carbon monoxide in the gaseous phase which is greater than 1:1.
The carbon monoxide may be used in the presence of other gases which are inert in the reaction. Examples of such gases include hydrogen, nitrogen, carbon dioxide and the noble gases such as argon.
The molar ratio of the ethylene to carbon monoxide in the gaseous phase, hereinafter termed the gaseous phase molar ratio, is greater than 1:1, preferably at least 3:1, particularly at least 5:1, especially from 5:1 to 50:1 and particularly especially from 7:1 to 15:1. Operating the process of the present invention with a gaseous phase molar ratio of less than 5:1, particularly of less than 3:1 and especially of less than 1:1 leads to a rapid deterioration in the performance of the catalyst.
It is believed that an important factor that influences the life of the catalyst system is the molar ratio of ethylene to carbon monoxide dissolved in the liquid phase, herein after termed the liquid phase molar ratio. The liquid phase molar ratio may differ from the gaseous phase molar ratio due to the different solubilities of ethylene and carbon monoxide in the liquid phase. The solubilities of ethylene and carbon monoxide in the liquid phase are dependent on factors such as the temperature, pressure and composition of the liquid phase. Consequently, in order to achieve the required liquid phase molar ratio, the gaseous phase molar ratio may need to be adjusted to compensate for such factors. Preferably, the gaseous phase molar ratio should be adjusted such that a liquid phase molar ratio of at least 5:1 is maintained.
The ratio of the number of moles of ethylene to the number of moles of carbon monoxide fed to the reactor by the ethylene feed stream and carbon monoxide feed stream in order to maintain the desired molar ratio of ethylene to carbon monoxide in the gaseous phase will depend on the reactor design. Where the gaseous phase is recycled after contact with the liquid phase then the ethylene and carbon monoxide feed streams are used to replenish the ethylene and carbon monoxide consumed during the carbonylation reaction and the ethylene and carbon monoxide that is removed with any offtake from the liquid phase. Thus, the ratio of the number of moles of ethylene to the number of moles of carbon monoxide fed by the feed streams is approximately 1:1. Alternatively, where the gaseous phase is not fully recycled then the ratio of the number of moles of ethylene to the number of moles of carbon monoxide fed by the feed streams will more closely match the desired molar ratio in the gaseous phase.
The feeds of ethylene and carbon monoxide may be continuous, intermittent or batch. Preferably, the initial feed into the reactor is of ethylene. This further reduces the poisoning of the catalyst system by the carbon monoxide.
The process of the present invention is preferably carried out at a temperature from 20 to 250xc2x0 C., in particular from 40 to 150xc2x0 C. and especially from 70 to 120xc2x0 C.
The process may be conducted under a total pressure of from 1xc3x97105 to 100xc3x97105 N.mxe2x88x922 and in particular from 5xc3x97105to 50xc3x97105 N.mxe2x88x922.
A preferred phosphine ligand is a bidentate phosphine of general formula (I) 
wherein
R0 is a tertiary carbon atom each of R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11 and R12 is independently a pendant optionally substituted organic group which carries a carbon atom through which the group is linked to the respective R0;
each of L1 and L2 is independently a linking group selected from an optionally substituted lower alkylene chain connecting the respective phosphorus atom to the group X; and
X is a bridging group comprising an optionally substituted aryl moiety to which the phosphorus atoms are linked on available adjacent carbon atoms.
The pendant optionally substituted organic groups of the preferred catalyst system, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10 and R11 and R12 may be independently selected from a wide range of components. Preferably, the pendant groups are optionally substituted lower alkyl, e.g. C1-8, and may be branched or linear.
Particularly preferred is when the organic groups, R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11 and R12 when associated with their respective R0 carbon atom, form composite groups which are at least as sterically hindering as t-butyl. Steric hindrance in this context is as discussed at page 14 et seq of xe2x80x9cHomogeneous Transition Metal Catalysisxe2x80x94A Gentle Artxe2x80x9d, by C Masters, published by Chapman and Hall, 1981.
The linking groups, L1 and L2, are independently selected from an optionally substituted, particularly lower alkylor lower alkylene, e.g. C1 to C4, chain. Especially preferred is when both L1 and L2 are methylene.
The bridging group X is an aryl moiety, e.g. a phenyl group, which may be optionally substituted, provided that the two phosphorus atoms are linked to adjacent carbon atoms, e.g. at the 1 and 2 positions on the phenyl group. Optional substitution of the aryl moiety may be by other organic groups, e.g. alkyl, particularly C1-8, aryl, alkoxy, carbalkoxy, halo, nitro, trihalomethyl and cyano. Furthermore, the aryl moiety may be a fused polycyclic group, e.g. naphthalene, biphenylene or indene.
Examples of suitable bidentate ligands are xcex1,xcex1xe2x80x2-bis(di-t-butyl phosphino)-o-xylene (also known as 1,2-bis(di-t-butylphosphinomethyl)benzene), xcex1,xcex1xe2x80x2-bis(dineopentyl phosphino)-o-xylene and 2,3-bis(di-t-butylphosphinomethyl)naphthalene. Additionally, the bidentate phosphine may be bonded to a suitable polymeric or inorganic substrate via at least one of the bridging group X, the linking group L1 or the linking group L2, e.g. xcex1,xcex1xe2x80x2-bis(di-t-butyl phosphino)-o-xylene may be bonded via the xylene group to polystyrene to give an immobile heterogeneous catalyst.
Preferably, the bidentate ligand is present in an amount such that the ratio of the number of moles of the bidentate ligand present to the number of moles of palladium present is from 1 to 50, e.g. 1 to 10 and particularly from 1 to 5 mol per mol.
Suitable compounds of palladium include salts of palladium with, or compounds comprising weakly co-ordinated anions derived from, nitric acid; sulphuric acid; lower alkanoic (up to C12) acids such as acetic acid and propionic acid including halogenated carboxylic acids such as trifluoroacetic acid and trichloroacetic acid; sulfonic acids such as methanesulfonic acid, chlorosulfonic acid, fluorosulfonic acid, trifluoromethanesulfonic acid, benzenesulfonic acid, naphthalene sulfonic acid, toluenesulfonic acids, e.g. p-toluenesulfonic acid, t-butylsulfonic acid, and 2-hydroxypropanesulfonic acid; sulfonated ion exchange resins; perhalic acids such as perchloric acid; halogenated carboxylic acids such as trichloroacetic acid and trifluoroacetic acid; orthophosphoric acid; phosphonic acids such as benzenephosphonic acid; and acids derived from interactions between Lewis acids and Broensted acids. Other sources which may provide suitable anions include the optionally halogenated tetraphenyl borate derivatives, e.g. perfluorotetraphenyl borate. Additionally, zero-valent palladium complexes particularly those with labile ligands, e.g. triphenylphosphine or alkenes such as dibenzylideneacetone or styrene may be used.
The anion may be introduced as one or more of an acid having a pKa measured in aqueous solution of less than 4, a salt with a cation that does not interfere with the reaction, e.g. metal salts or largely organic salts such as alkyl ammonium, and a precursor, such as an ester, that can break down under reaction conditions to generate the anion in situ. Suitable acids and salts include the acids and salts, other than unsubstituted carboxylates, listed supra.
The quantity of anion present is not critical to the catalytic behaviour of the catalyst system. The molar ratio of anion to palladium may be from 1:1 to 500:1, preferably from 2:1 to 100:1 and particularly from 3:1 to 30:1. Where the anion is provided by a combination of acid and salt, the relative proportion of the acid and salt is not critical.
The catalyst system may be used homogeneously or heterogeneously. Preferably the catalyst system is used homogeneously.
The catalyst system is preferably constituted in the liquid phase which may be formed by one or more of the reactants or by the use of a suitable solvent.
Suitable solvents that may be used in conjunction with the catalyst system include one or more aprotic solvents such as ethers, e.g. diethyl ether, dimethyl ether of diethylene glycol, anisole and diphenyl ether, aromatic compounds, including halo variants of such compounds, e.g. benzene, toluene, ethyl benzene, o-xylene, m-xylene, p-xylene, chlorobenzene, odichlorobenzene, mdichlorobenzene, and p-dichlorobenzene; alkanes, including halo variants of such compounds, e.g. hexane, heptane, 2,2,3-trimethylpentane, methylene chloride and carbon tetrachloride; nitriles, e.g. benzonitrile and acetonitrile; esters, e.g. methyl benzoate, methyl acetate, methyl propionate and dimethyl phthalate; sulfones, e.g. diethyl sutfone and tetrahydrothiophene 1,1- dioxide; carboxylic acids, e.g. propionic acid It is preferred to use as a solvent a compound which takes part in the reaction either as a reactant or a product, to minimise the number of different compounds present in the liquid phase to facilitate separation of the mixture. Therefore in the example when ethylene and carbon monoxide are carbonylated in the presence of methanol to form methyl propionate, a particularly suitable solvent is methyl propionate.
The end product of the reaction is determined at least in part by the source of hydroxyl groups that is used. The use of water gives rise to the corresponding carboxylic acid whereas the use of an alkanol leads to the corresponding ester. Suitable alkanofs include C1-30 alkanols, optionally substituted with one or more substituents such as halogen atoms, cyano, carbonyl, alkoxy or aryl groups. Suitable alkanois include one or more of methanol, ethanol, propanol, 2-propanol, 2-butanol, t-butyl alcohol and chlorocapryl alcohol. Particularly useful are methanol and ethanol. Additionally or alternatively, polyhydroxyl compounds, such as diols and sugars, may be used.
The molar ratio of the amount of ethylene used in the reaction to the amount of hydroxyl providing compound is not critical and may vary between wide limits, e.g. from 0.001:1 to 100:1 mol/mol in the liquid phase.
The process of the invention is particularly suitable for the production of methyl propionate. In one preferred embodiment therefore we provide a process for the production of methyl propionate comprising the steps of
(i) forming a gaseous phase from an ethylene feed stream and a carbon monoxide feed stream;
(ii) contacting the gaseous phase with a catalyst system within a liquid phase comprising methanol, a solvent and a catalyst system comprising palladium, or a compound thereof, a phosphine ligand and a source of anions; and
(iii) reacting the ethylene with the carbon monoxide in the presence of the methanol and of the catalyst system;
characterised in that the ethylene feed stream and carbon monoxide feed stream provide a molar ratio of ethylene to carbon monoxide in the gaseous phase which is greater than 1:1.
Preferred catalyst systems are those described above. Preferably the solvent comprises methanol, methyl propionate or a mixture thereof.